The function of the nervous system is dependent on complex interactions between networks of neurons composed of multiple neuron types. Understanding how these networks function both in health and disease is dependent on understanding the precise connectivity between specific neurons types. It is therefore apparent that, in order to have an adequate understanding of the nervous system, it is necessary to have detailed descriptions of neuronal connectivity with the same level of precision at which these systems operate. The research proposed here is aimed at revealing the detailed connectivity of specific types of inhibitory cortical neurons. A novel Cre-dependent and rabies-based tracing system will be used to reveal the direct monosynaptic connections to neurons in primary somatosensory cortex of five different Cre-driver mouse lines. The intersection of Cre-expression in specific cell groups, with AAV infection and specific promoters, will define specific populations of inhibitory neurons that can be infected with the rabies virus. Transcomplementation and retrograde transsynaptic spread of the rabies virus will reveal the cells, across the entire brain, that make direct connections to the targeted cortical inhibitory neurons. To assess the specificity of local outputs from specific types of inhibitory cortical neurons, similar targeting methods will be used to selectively express channelrhodopsin (ChR2) in specific cell groups. Living brain slices will then be prepared and functional connections assayed by recording intracellularly from potential postsynaptic cell types of interest while "optogenetically" activating the ChR2 positive cells. The results of these experiments will shed light on the feasibility of various hypotheses about the functional roles of particular types of inhibitory cortical neurons and will guide future studies monitoring and manipulating the activity of specific cell types. PUBLIC HEALTH RELEVANCE: Understanding the detailed organization of cortical circuits involving specific inhibitory neuron types is necessary to obtain a mechanistic understanding of the function of the cerebral cortex. Understanding the specific roles of inhibitory neurons in cortical function has important implications for human health, as these cell types and their activities are implicated in the cortical mechanisms that regulate attention and their disruption is implicated in schizophrenia and autism.